A flat panel display possesses advantages of being ultra thin, power saved and radiation free and has been widely utilized. Present flat panel displays mainly comprise a LCD (Liquid Crystal Display) or an OLED (Organic Light Emitting Display).
An organic light emitting diodes display possesses outstanding properties of self-illumination, no required back light, high contrast, being ultra thin, wide view angle, fast response, being applicable for flexible panel, wide usage temperature range, simple structure and manufacture process and etc., and therefore, it is considered to be a new applicable technology for the next generation flat panel display.
Thin film transistors (TFTs) are important components of a flat panel display, which can be formed on a glass substrate or a plastic substrate. Generally, the thin film transistors are employed as switch elements and driving elements utilized in such as LCDs, OLEDs, electrophoresis displays (EPD).
The oxide semiconductor TFT technology is the most popular technique at present. Because oxide semiconductor has higher electron mobility and simpler manufacture process in comparison with low temperature poly-silicon (LTPS) and has higher compatibility with the amorphous silicon process. Therefore, the oxide semiconductor has been widely utilized in the technical field of large scale organic light emitting display and has the great opportunity of application development.
In the present oxide semiconductor TFT backplate structure, an etching stopper layer structure, a back channel etching structure, a coplanar structure, and et cetera have been well developed and provide more applications. These structures have respective advantages and drawbacks of their own. For example, the etching stopper layer structure comprises an etching stopper layer to protect the oxide semiconductor layer. The stability is better but the manufacture of the etching stopper layer needs one extra mask and the coupling capacitance is larger which goes against the promotion of yield and decrease of manufacture cost. The back channel etching structure and the coplanar structure can eliminate one mask in the manufacture process to reduce the manufacture cost and the corresponding coupling capacitance can be smaller, which provides higher competitiveness and development prospects. However, the stabilities of these two structures have yet to be promoted.
Furthermore, the present oxide semiconductor TFT backplate generally comprises a switch TFT and a drive TFT. In the traditional process, the switch TFT and the drive TFT are formed by the same manufacture process in general. Therefore, the switch TFT and the drive TFT have the same structure and the same electrical properties, such as the same conducting current (Ion), threshold voltage (Vth), subthreshold swing (S.S) and et cetera. Nevertheless, the demands of the electrical properties for the switch TFT and the drive TFT are different in practical usages. In general, the switch TFT is expected to have a smaller S.S to achieve the object of fast charge and discharge. The drive TFT is expected to have a slightly larger S.S for controlling the current and the grey scale more precisely.